Synthesis of (E)- and (Z)-α,β-Difluorourocanic Acid

Abstract

Horner-Emmons fluoroolefination of an aryl aldehyde followed by introduction of a second fluorine via “FBr” addition provides an original approach to the preparation of 1-alkyl-2-aryl-1,2-difluoroethenes. The utility of this procedure is demonstrated by the preparation of (E and Z)-α,β-difluorourocanic acid.

1. Introduction

(E)-Urocanic acid 1a is elaborated in vivo by histidine ammonia lyase-catalyzed loss of ammonia from histidine. The photochemistry and biological properties of urocanic acid continue to receive attention, in part because of evidence that (Z)-urocanic acid (1b), formed in the body by photo-isomerization of the E isomer, is a mediator of photo-immunosuppression [1]. As part of our program to prepare fluorinated analogues of biologically important imidazoles, we have reported the synthesis of E- and Z- 2- and 4-fluorourocanic acid (2a,b and 3a,b, respectively) [2], E- and Z-α-fluorourocanic acid (4a,b) [3] and E- and Z-β-fluorourocanic acid (5a,b) [4]. Ring-fluorinated analogues 2 and 3 were prepared from ring-fluorinated aldehyde precursors using a Horner-Emmons olefination with triethyl phosphonoacetate [2]. A similar olefination of 1-trityl-(1H)imidazole-4-carboxaldehyde with triethyl flourophosphonoacetate was the key step in the synthesis of 4a,b [3]. To access the β-fluoroanalogues 5a,b, addition of “FBr” to a vinyl imidazole derivative to place fluorine adjacent to the imidazole ring was the key step [4]. Missing from the inventory of side-chain fluorinated analogues of urocanic acid is α,β-difluorourocanic acids (6a, 6b), compounds that should have interesting chemical and biological behavior by virtue of the difluoroacrylate functionality.

Published routes to compounds of the general structure Aryl-CF=CF-Alkyl would not seem readily applicable to the synthesis of 6a,6b. One approach based on reaction of CF₂=CF₂ or R-CF=CFX (where X is H or halogen) with ArLi has been applied to heteroaryl compounds [5]. However, in general this method would not readily tolerate functional groups. A second approach that involves elimination “X-X” from R-CFX-CFX-R [6] requires lengthy and, in our case, problematic preparation of precursors. A third approach would be direct addition of F₂ diluted with N₂ to a triple bond. In one example, addition of F₂ to a series of tolanes give mixtures of products, including α,α’-difluorostilbenes [7]. We did not consider this approach because of the complex reaction products and requirement for special equipment.

We have approached the synthesis of 6a,b by combining chemistry we used for the preparation of 4 and 5. In a key reaction in our synthesis of α-fluorourocanic acids, Horner-Emmons olefination of 1-trityl-(1H)imidazole-4-carboxaldehyde with triethyl fluorophosphonoacetate produces α-fluoropropenoates 7 [3]. In our synthesis of β-fluorourocanic acids we took advantage of regiospecific “FBr” addition to a vinyl imidazole derivative followed by HBr elimination to install fluorine in the β-position [4]. In contemplating a combination of these two approaches in order to place fluorine at both the α- and β-positions, a critical question involves regioselectivity of the “FBr” addition to 1-alkyl-2-aryl-1-fluoroalkenes. If the directive effect of fluorine (resulting in stabilization of an α-carbonium ion) is strong enough to take precedence over aryl carbonium ion stabilization, “FBr” addition would be predicted to lead to the geminal instead the desired vicinal difluoro compounds. Indeed, this is the observed product of addition of FBr to vinyl fluorides where an aryl group is not present [8] or when the aryl group and fluorine are on the same carbon [9]. To our surprise, we have found no examples of electrophilic additions to 1-alkyl-2-aryl-1-fluoroalkenes.

Having found no precedent wherein addition of “FBr” to a vinyl monofluoride produces the vicinal difluoride we were pleased to find that addition of “FBr” to 3-(1-trityl-1(H)-imidazol-4-yl)-1-hydroxymethyl-2-fluoro-2-propene (8) in fact proceeds with regioselectivity to produce the vicinal difluoride. This somewhat unexpected direction of addition provides a convenient route to α,β-difluorourocanic acids. In addition, this discovery should make possible the synthesis of other 1-aryl-1,2-difluoroalkenes by the same route.

2. Chemistry

(E- And Z)-ethyl 2-fluoro-3-(1-trityl-1-H-imidazol-4-yl)-prop-2-enoate (7a,b) were prepared as described [4]. Based on our experience with the preparation of 5, we were aware that reduction of the ester to alcohol was necessary to achieve the desired chemoselectivity of halide elimination (bromide vs. fluoride) in the sequence [4]. Previously we also found that the carboxyl function of urocanic acid deactivates the double bond to the extent that “FBr” addition via Et₃N·3HF and NBS becomes a sluggish process [4]. In a related example, we were unable to effect “FBr” addition to ethoxycabonylalkynylimidazole [10]. Consistent with these results, we found that no FBr-adducts are formed from α-bromourocanate or α-fluorourocanate 7 using our usual conditions. Therefore we reduced the deactivating ester function to the hydroxymethyl electron donating group to obtain 2-fluoropropenols 8a and 8b. These were then used as substrates to study FBr adduct formation. In subsequent reactions, since addition of “FBr” to either olefin produced diasteromeric mixtures of products that were extremely difficult to separate, it was convenient to carry out addition of “FBr” and elimination of HBr on mixtures of 8a/8b.

The reactivity of the 2-fluoropropenols 8a,b proved to be lower in comparison to their nonfluorinated analogs [4] or isomeric 3-fluoropropenols [9], where the conversion is about 100%. Under the usual conditions, the conversion of 2-fluoropropenols to products 9a and 9b was only 84% of 8a and 52% of 8b. As is usual in the cases of poorly reactive olefins, in addition to the desired FBr adducts, the dibromo adducts 10 were also formed in about 25% yield. Thus the reactivity is similar to trityl urocanic methyl ester (conversion 62%) [4]. As in that case, the conversion of 8 can be increased by increasing the reagent amounts (see Table 1). We found no evidence (¹H and ¹⁹F NMR) of reversed regioselectivity with formation of geminal difluoro compound.

Table 1.

Results of addition of “FBr” on 6a or 6b

	Starting Isomer	Reaction conditionsa			Products molar ratio [%]
		“HF”	NBS	Time	9a	9b	10a	10b	8b
a)	8a	1.5	1.1	2	41	17	13	13	16
b)	8a	1.5	1.1	5	51	18	6	6	19
c)	8b	1.5	1.1	14	3	24	12	13	48
d)	8b	2.5	1.8	44	6	41	11	15	27
e)	8b	3.2	2.2	19	8	59	15	18	0

^acontent of columns is as follows: equivalents of Et₃N·3HF, equivalents of NBS, reaction time in hours

^bstarting isomer 8a or 8b.

It is well documented that in these reactions, trans alkenes give mainly the product of anti addition, but cis alkenes give mixture of anti and syn addition [4]. However, there are few precedents for reactions of alkenes having a halogen substituent in place of hydrogen on the double bond. We found that isomer 8b, where the carbon substituents are trans to each other, gives diastereoisomeric adducts in a ratio of 88:12, tentatively assigned as products from anti (9b, 2R^*,3S^*-configuration) and syn (9a, 2R^*,3R^*-configuration) additions, respectively. This is slightly lower diastereoselectivity than found with the nonfluorinated analog (95:5) [4]. Isomer 8a, where the carbon substituents are in a cis orientation, gives a diastereoisomeric mixture in a ratio of 75:25, tentatively assigned as products from anti (9a) and syn (9b) additions. This is significantly higher anti diastereoselectivity than found with the cis-configured nonfluorinated analog which gave an anti/syn addition ratio of 20:80 [4]. It is interesting to note that byproduct Br₂-adducts, two diastereoisomers 10a and 10b are formed in ratio of about 1:1 in both cases. These results are summarized in the Table 1.

Dehydrobromination of mixture 9a and 9b gives corresponding difluoropropenols 11a and 11b in the usual yields of 60-70%. Double bond configuration was readily assigned to the two geometrical isomers based on ¹⁹F-NMR data. The (E)-isomer 11a has a significantly higher F-F coupling constant (126 Hz) than does the (Z)-isomer 11b (21 Hz). The oxidation to aldehyde 12a and 12b by MnO₂ surprisingly proceeds with formation of byproduct 13, the yield of which increases with longer reaction times or increased reaction temperature. The isomer 11b is significantly less reactive than 11a and compound 13 becomes the main product of the oxidation (see Exp. 4.8). We have found no precedent for this conversion and any mechanistic proposals would be entirely speculative. For acceptable yield of the aldehydes 12 (30-50%) it is necessary to use a large excess of MnO₂ and to monitor the reaction by TLC in order to stop the reaction immediately after full conversion of the starting material.

Subsequent one pot oxidation and deprotection of aldehydes 12 afforded the desired α,β-difluorourocanic acids 6a and 6b. We did not purify and characterize the immediate products of the oxidation due to complications with their purification that led to decomposition. This was especially true for the product of oxidation of aldehyde 10b. By this one pot procedure, we obtained (E)-α,β-difluorourocanic acid (6a) in high yield (96 %) and (Z)-α,β-difluorourocanic acid (6b) in moderate yield (37 %).

Scheme 1 - a) DIBAL-H, CH₂Cl₂, 74 % 8a, 72 % 8b; b) Et₃N·3HF, NBS, CH₂Cl₂, 58 % 9a,b; c) Et₃N, DMSO, 47 % 11a, 29 % 11b; d) MnO₂, CH₂Cl₂, 53 % 12a, 33 % 12b; e) NaHPO₄, NaClO₂, t-BuOH/H₂O; f) HCl, CH₃CO₂H, 96 % 6a, 37 % 6b.

3. Conclusion

We report here the synthesis new fluorinated analogues of urocanic acid. The methodology developed includes a procedure for the preparation of functionalized 1-alkyl-2-aryl-1,2-difluoroethenes from readily available starting materials. For this synthesis there was no requirement for hydroxyl group protection. This procedure should be applicable to other aryl and heteroaryl systems as borne out by our preliminary successful experiments with benzaldehydes. Studies on the biology and photochemistry of fluorinated urocanic acids are a subject of continued research. Neither 6a or 6b was an inhibitor of urocanase.

4. Experimental

4.1. General

The NMR spectra were recorded at 22°C on a Varian Mercury-300 instrument at frequencies of 300.1 for ¹H, 75.5 for ¹³C and 282.2 MHz for ¹⁹F spectra. All ¹³C-NMR spectra are proton-decoupled. Tetramethylsilane (TMS) was used as the internal standard; ¹³C NMR chemical shifts: CDCl₃, δ = 77.23; DMSO-d₆, δ = 39.51 ppm. For ¹⁹F NMR, fluorotrichloromethane was used as the internal standard. HRMS spectra were recorded on a Waters LCT Premier TOF mass spectrometer. Flash chromatography was performed on Biotage SP4. LC/MS was performed on Agilent 1100 system.

4.2. (E)-Ethyl 2-fluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-enoate (7a)

Compound 7a was prepared as published [4]. We report here additional spectral data. ¹³C NMR (CDCl₃): 160.68 (d, ²J_CF = 34.1, CO), 144.76 (d, ¹J_CF = 250.2, CF), 142.01 (3C, Tr), 138.78 (s, C2_Imi), 131.56 (d, ³J_CF = 11.2, C4_Imi), 129.63 (6CH, Tr), 128.06 (3CH, Tr), 128.03 (6CH, Tr), 125.75 (d, ⁴J_CF = 5.4, C5_Imi), 117.23 (d, ²J_CF = 30.8; CH=CF), 75.70 (s, 1C, Tr), 61.07 (s, CH₂), 13.96 (s, CH₃).¹⁹F NMR (CDCl₃): -124.3 (d, ³J_HF = 24.0).

4.3. (Z)-Ethyl 2-fluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-enoate (7b)

Compound 7b was prepared as published [4]. We report here additional spectral data.¹³C NMR (CDCl₃): 161.09 (d, ²J_CF = 33.8, CO), 145.83 (dd, ¹J_CF = 259.7, CF), 141.88 (3C, Tr), 139.47 (s, C2_Imi), 132.43 (d, ³J_CF = 3.32, C4_Imi), 129.66 (6CH, Tr), 128.25 (3CH, Tr), 128.16 (6CH, Tr), 125.13 (d, ⁴J_CF =13.4, C5_Imi), 112.73 (d, ²J_CF = 8.2, CH=CF), 75.88 (s, 1C, Tr), 61.45 (s, CH₂), 14.15 (s, CH₃). ¹⁹F NMR (CDCl₃): -123.9 (dd, ³J_HF = 36.2, ⁵J_HF = 1.6).

4.4. (E)-and (Z)-2-Fluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-en-1-ol (8a,b)

To a stirred solution of propenoates 7 (6.37 g, 14.9 mmol, 7a:7b 5:9) in 370 mL of dry CH₂Cl₂ was slowly added 33 mL of DIBAL-H (1M in CH₂Cl₂, 33 mmol) at -65 °C. The mixture was allowed to warm to room temperature and was stirred overnight. After addition of 30 mL of water, the mixture was partitioned between CH₂Cl₂ and water/brine. The organic part was dried over MgSO₄ and evaporated to dryness. The resulting solid was separated by column chromatography (250 g, CH₂Cl₂:Et₂O, 9:1) to give pure 8a (1.51 g, 74% based on 7a). Isomer 8b was eluted with CH₂Cl₂:CH₃OH (9:1) and purified by column chromatography (100 g, CH₂Cl₂:EtOAc 1:1) to give pure 8b (2.65 g, 72% based on 7b).

4.4.1. (E)-2-Fluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-en-1-ol (8a)

¹H NMR (CDCl₃): 7.42 (1H, d, J = 1.5, Imi), 7.38-7.32 (9H, m, Tr), 7.17-7.09 (6H, m, Tr), 7.04 (1H, bs, Tr), 6.74 (1H, d, J = 1.5, Imi), 6.05 (1H, d, ³J_HF = 20.8, CH=CF), 4.44 (2H, d, ³J_HF = 14.8, CH₂).¹³C NMR (CDCl₃): 162.09 (d, ¹J_CF = 259.3, CF), 141.87 (3C, Tr), 138.77 (s, C2_Imi), 134.30 (d, ³J_CF = 15.1, C4_imi), 129.64 (6CH, Tr), 128.21 (3CH, Tr), 128.13 (6CH, Tr), 120.24 (d, ⁴J_CF = 8.1, C5_Imi), 101.69 (d, ²J_CF = 30.8, CH=CF), 75.57 (s, 1C, Tr), 59.97 (d, ²J_CF = 37.9, CH₂). ¹⁹F NMR (CDCl₃): -102.7 (dt, ³J_HF = 20.7, ³J_HF = 15.0). Mp: 194.5-195.5 °C (CH₂Cl₂/Et₂O). HRMS: for M+H, i.e. C₂₅H₂₂FN₂O, calcd 385.1716; found 385.1729. Elemental analysis for C₂₅H₂₁FN₂O calcd: 78.10 %C, 5.51 %H, 7.29 %N, found: 77.99 %C, 5.51 %H, 7.32 %N.

4.4.2. (Z)-2-Fluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-en-1-ol (8b)

¹H NMR (CDCl₃): 7.39 (1H, d, J = 1.4, Imi), 7.34-7.28 (9H, m, Tr), 7.16-7.10 (6H, m, Tr), 7.09 (1H, t, J = 1.6, Imi), 6.01 (1H, d, ³J_HF = 40.1, CH=CF), 5.24 (1H, bs, OH), 4.16 (2H, d, ³J_HF = 12.6, CH₂). ¹³C NMR (CDCl₃): 158.86 (d, ¹J_CF = 264.2, CF), 142.07 (3C, Tr), 138.04 (s, C2_Imi), 133.50 (d, ³J_CF = 1.0, C4_imi), 129.71 (6CH, Tr), 128.09 (3CH, Tr), 128.07 (6CH, Tr), 121.31 (d, ⁴J_CF = 11.7, C5_Imi), 101.07 (d, ²J_CF = 9.7, CH=CF), 75.58 (s, 1C, Tr), 60.30 (d, ²J_CF = 32.2, CH₂). ¹⁹F NMR (CDCl₃): -109.7 (dt, ³J_HF = 40.1, ³J_HF = 12.6). Mp: 194-195 °C (ethyl acetate). Elemental analysis for C₂₅H₂₁FN₂O calcd: 78.10 %C, 5.51 %H, 7.29 %N, found: 77.98 %C, 5.53 %H, 7.32 %N.

4.5. 2-Bromo-2,3-difluoro-3-(1-trityl-1H-imidazol-4-yl)-propan-1-ol (9a,b)

To a solution of compound 8a,b (3.11 g, 8.1 mmol) in 40 mL of anhydrous dichloromethane was slowly added Et₃N·3HF (1.94 g, 12.0 mmol) at 0 °C. After the solution was stirred for 10 min, 1.59 g of NBS (8.9 mmol) was added. The cooling bath was removed and the reaction was stirred for 2 days at which time TLC indicated that almost all the starting material had disappeared. The solvent was evaporated and 150 mL of ethyl acetate was added. The resulting solution was washed with water (2 × 20 mL), brine (2 × 20 mL) and dried over anhydrous Na₂SO₄. Evaporation of the solvent and purification of the residue by chromatography on silica gel (hexane/ethyl acetate 2:1) afforded 2.27 g (58 %) of a mixture of compound 9a and compound 9b as a white solid. ¹HNMR indicated the ratio of the two compounds to be 40:60. HRMS (CI⁺): for M+H i.e. C₂₅H₂₂BrF₂N₂O calcd: 483.0884; found: 483.0833.

4.5.1 (2R^*,3R^*)-2-Bromo-2,3-difluoro-3-(1-trityl-1H-imidazol-4-yl)-propan-1-ol (9a)

¹H NMR (CDCl₃): 7.47 (1H, d, J = 1.4, Imi), 7.35 (9H, m, Tr), 7.11 (6H, m, Tr), 7.07 (1H, dd, J = 3.2, 1.4, Imi), 6.24 (1H, bs, OH), 5.77 (1H, dd, ²J_HF = 45.9, ³J_HF = 10.6), 4.44 (1H, ddd, ²J_HH = 13.0, ³J_HF = 5.4, ⁴J_HF = 1.4), 3.94 (1H, dm, ²J_HH = 14.2).¹³C NMR (CDCl₃): 141.63 (3C, Tr), 139.15 (s, C2_Imi), 133.23 (dd, ²J_CF = 22.6, ³J_CF = 7.1, C4_Imi), 129.66 (6CH, Tr), 128.43 (3CH, Tr), 128.27 (6CH, Tr), 123.79 (dd, ³J_CF = 5.9, ⁴J_CF = 1.2, C5_Imi), 109.68 (dd, ¹J_CF = 260.0, ²J_CF = 26.4, CFBr), 89.86 (dd, ¹J_CF = 183.9, ²J_CF = 26.5, CHF), 76.03 (s, 1C, Tr), 65.08 (dd, ³J_CF = 31.0, ⁴J_CF = 2.2, CH₂). ¹⁹F NMR (CDCl₃): -120 (1F, m), -169.4 (1F, dd, ²J_HF = 45.8, ³J_FF = 22.2).

4.5.2 (2R*,3S*)-2-Bromo-2,3-difluoro-3-(1-trityl-1H-imidazol-4-yl)-propan-1-ol (9b)

¹H NMR (CDCl₃): 7.46 (1H, t, J = 1.5, Imi), 7.35 (9H, m, Tr), 7.11 (6H, m, Tr), 7.05 (1H, dt, J = 2.3, 1.0, Imi), 6.24 (1H, bs, OH), 5.87 (1H, ddd, ²J_HF = 45.0, ³J_HF = 9.5, ⁴J_HH = 1.0), 4.17 (1H, dd, ²J_HH = 13.4, ³J_HF = 3.5), 4.04 (1H, dd, ³J_HF = 26.9, ²J_HH =13.4). ¹³C NMR (CDCl₃): 141.74 (3C, Tr), 138.75 (s, C2_Imi), 134.12 (dd, ²J_CF = 25.8, ³J_CF = 7.3, C4_Imi), 129.66 (6CH, Tr), 128.36 (3CH, Tr), 128.23 (6CH, Tr), 121.81 (dd, ³J_CF = 5.0, ⁴J_CF = 3.3, C5 _Imi), 110.64 (dd, ¹J_CF = 263.7, ²J_CF = 22.6, CFBr), 90.18 (dd, ¹J_CF = 182.2, ²J_CF = 26.9, CHF), 76.01 (s, 1C, Tr), 67.81 (dd, ³J_CF = 23.7, ⁴J_CF = 2.5, CH₂). ¹⁹F NMR (CDCl₃): -120 (1F, m), -189.1 (1F, dd, ²J_HF = 45.3, ³J_FF = 21.6).

4.5.3. 2,3-Dibromo-2-fluoro-3-(1-trityl-1H-imidazol-4-yl)-propan-1-ol (10a)

¹H NMR (CDCl₃): 7.46 (1H, dd, J = 1.4, 0.4, Imi), 7.37-7.32 (9H, m, Tr), 7.15-7.09 (6H, m, Tr), 6.98 (1H, d, J = 1.4, Imi), 6.36 (1H, bs, OH), 5.63 (1H, dd, ³J_HF = 8.1, ⁴J_HH = 0.9), 4.63 (1H, dd, ²J_HH = 13.0, ³J_HF = 4.5), 3.94 (1H, ddd, ²J_HH = 13.0, ³J_HF = 12.2, ⁴J_HH = 1.0). ¹³C NMR (CDCl₃): 141.48 (3C, Tr), 139.04 (s, C2_Imi), 135.88 (d, ³J_CF = 7.0, C4_Imi), 129.60 (6CH, Tr), 128.40 (3CH, Tr), 128.22 (6CH, Tr), 122.05 (d, ⁴J_CF = 1.9, C5_Imi), 110.85 (d, ¹J_CF = 269.9, CF), 76.05 (s, 1C, Tr), 66.55 (d, ²J_CF = 30.9, CHBr), 51.49 (d, ²J_CF = 28.1, CH₂). ¹⁹F NMR (CDCl₃): -105.28 (1F, bs).

4.5.4. 2,3-Dibromo-2-fluoro-3-(1-trityl-1H-imidazol-4-yl)-propan-1-ol (10b)

¹H NMR (CDCl₃): 7.44 (1H, d, J = 1.5, Imi), 7.37-7.32 (9H, m, Tr), 7.15-7.09 (6H, m, Tr), 7.08 (1H, m, Imi), 6.36 (1H, bs, OH), 5.61 (1H, dd, ³J_HF = 11.2, ⁴J_HH = 0.7), 4.22 (1H, dd, ²J_HH = 13.1, ³J_HF = 11.5), 4.07 (1H, dd, ³J_HF = 23.5, ²J_HH = 13.1). ¹³C NMR (CDCl₃): 141.62 (3C, Tr), 139.10 (s, C2_Imi), 136.39 (d, ³J_CF = 4.5, C4_Imi), 129.60 (6CH, Tr), 128.32 (3CH, Tr), 128.18 (6CH, Tr), 123.17 (d, ⁴J_CF = 3.4, C5_Imi), 112.57 (d, ¹J_CF = 265.5, CF), 76.05 (s, 1C, Tr), 68.34 (d, ²J_CF = 24.9, CHBr), 49.68 (d, ²J_CF = 26.2, CH₂). ¹⁹F NMR (CDCl₃): -111.73 (1F, bs).

4.5.5 Reaction of “FBr” with pure (E)-2-Fluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-en-1-ol (8α)

To a solution of compound 8a (1.44 g, 3.7 mmol) in 40 mL of anhydrous dichloromethane was slowly added Et₃N·3HF (0.94 g, 5.8 mmol) at 0 °C. After the solution was stirred for 30 min., 0.73 g of NBS (4.1 mmol) was added. The cooling bath was removed after 60 min. and the reaction was stirred for an additional 4 hours at which time TLC indicated that almost all the starting material had disappeared. The solvent was evaporated and 70 mL of ethyl acetate was added. The resulting solution was washed with water (2 × 10 mL), brine (2 × 10 mL) and dried over anhydrous Na₂SO₄. Evaporation of the solvent and purification of the residue by chromatography on silica gel (dichloromethane/ethyl acetate 9:1) afforded 1.00 g (55 %) of a mixture of compound 9a and compound 9b as a white solid. ¹HNMR indicated the ratio of the two compounds to be 75:25.

4.5.6 Reaction of “FBr” with pure (Z)-2-Fluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-en-1-ol (8b)

To a solution of compound 8b (870 mg, 2.26 mmol) in 40 mL of anhydrous dichloromethane was slowly added Et₃N·3HF (547 mg, 3.39 mmol) at 0 °C. After the solution was stirred for 30 min., 445 mg of NBS (2.50 mmol) was added. The cooling bath was removed after 60 min. and the reaction was stirred for an additional 2 days at which time TLC indicated that almost all the starting material had disappeared. The solvent was evaporated and 70 mL of ethyl acetate was added. The resulting solution was washed with water (2 × 10 mL), brine (2 × 10 mL) and dried over anhydrous Na₂SO₄. Evaporation of the solvent and purification of the residue by chromatography on silica gel (dichloromethane/ethyl acetate 1:1) afforded 226 mg (18 %) of a mixture of compound 9b and compound 9a as a white solid. ¹HNMR indicated the ratio of the two compounds to be 88:12.

4.6. (E)-and (Z)-2,3-Difluoro-3-(1-trityl-1-H-imidazol-4-yl)-prop-2-en-1-ol (11a and 11b)

In a 100 mL flask, 1.51 g of a mixture of compounds 9a and 9b (3.13 mmol) was dissolved in 50 mL of anhydrous DMF and 5 mL of triethylamine (~ 36 mmol) at room temperature. The mixture was heated to 100 °C and stirred overnight. After evaporation of the solvent, the residue was dissolved in 200 ml of ethyl acetate. The resulting mixture was washed with water (2 × 30 mL), brine (2 × 30 mL) and dried over Na₂SO₄. The solvent was evaporated and the residue was purified by chromatography on silica gel (hexane/ethyl acetate 2:1) to give 585 mg of compound 11a (47 %) and 365 mg of compound 11b (29 %) as white solids. The ratio 11a:11b in the crude mixture was 60:40 (calculated from LC and ¹⁹F-NMR).

4.6.1 (E)-2,3-Difluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-en-1-ol (11a)

¹H NMR (CDCl₃): 7.51 (1H, bs, Imi), 7.36-7.31 (9H, m, Tr), 7.16-7.10 (7H, m, Tr, Imi), 4.46 (2H, dd, ³J_HF = 22.9, ⁴J_HF = 5.5), 3.21 (1H, bs, OH). ¹³C NMR (CDCl₃): 148.65 (dd, ¹J_CF = 244.9, ²J_CF = 50.0, CF), 144.44 (dd, ¹J_CF = 228.7, ²J_CF = 46.8, CF), 141.86 (3C, Tr), 139.33 (s, C2_Imi), 130.23 (dd, ²J_CF = 29.0, ³J_CF = 6.1, C4_Imi), 129.66 (6CH, Tr), 128.26 (3CH, Tr), 128.18 (6CH, Tr), 121.39 (dd, ³J_CF = 11.1, ⁴J_CF = 5.6, C5_Imi), 75.85 (s, 1C, Tr), 55.88 (d, ²J_CF = 24.3, CH₂). ¹⁹F NMR (CDCl₃): -156.0 (1F, dt, ³J_FF = 125.5, ³J_HF = 22.9), -161.6 (1F, dt, ³J_FF = 125.5, ²J_HF = 5.3). HRMS (FAB⁺): for MH, i.e. C₂₅H₂₁F₂N₂O, calcd. 403.1622; Found: 403.1623. Mp. 157-158 °C (ethylacetate). Elemental analysis for C₂₅H₂₀F₂N₂O calcd: 74.61 %C, 5.01 %H, 6.96 %N, found: 74.50 %C, 5.14 %H, 6.91 %N.

4.6.2. (Z)-2,3-Difluoro-3-(1-trityl-1H-imidazol-4-yl)-prop-2-en-1-ol (11b)

¹H NMR (CDCl₃): 7.47 (1H, dd, J = 2.8, 1.5, Imi), 7.38-7.33 (9H, m, Tr), 7.16-7.70 (6H, m, Tr), 7.06 (1H, bs, Imi), 5.25 (1H, bs, OH), 4.51 (2H, dd, ³J_HF = 20.8, ⁴J_HF = 4.1, CH₂). ¹³C NMR (CDCl₃): 146.84 (dd, ¹J_CF = 258.1, ²J_CF = 13.3, CF), 142.30 (dd, ¹J_CF = 237.7, ²J_CF = 19.8, CF), 141.71 (3C, Tr), 139.44 (d, ⁴J_CF = 1.7, C2_Imi), 131.60 (dd, ²J_CF = 33.7, ³J_CF = 1.4, C4_Imi), 129.65 (6CH, Tr), 128.41 (3CH), 128.28 (6CH), 119.95 (dd, ³J_CF = 9.3, ⁴J_CF = 1.0, C5_Imi), 76.03 (s, 1C, Tr), 58.27 (dd,²J_CF = 29.0, ³J_CF = 2.2, CH₂). ¹⁹F NMR (CDCl₃): -136.1 (1F, td, ³J_HF = 20.7, ³J_FF = 10.3), -147.6 (1F, dtd, ³J_FF = 10.3, ⁴J_HF = 3.8, ⁴J_HF = 3.2). HRMS (FAB⁺): for MH, i.e. C₂₅H₂₁F₂N₂O, calcd. 403.1622; found: 403.1630. Mp: 140-141 °C (ethylacetate). Elemental analysis for C₂₅H₂₀F₂N₂O calcd: 74.61 %C, 5.01 %H, 6.96 %N, found: 74.44 %C, 4.90 %H, 6.90 %N.

4.7. (E)-α,β-Difluoro-3-(1-trityl-1H-imidazol-4-yl)-propenal (12a)

To a solution of compound 11a (380 mg, 0.95 mmol) in 10 mL of anhydrous dichloromethane (20 mL) was added activated MnO₂ (804 mg, 9.25 mmol). The mixture was stirred at room temperature for 12 hours until TLC indicated complete disappearance of starting material. The mixture was filtered and solvent was evaporated. The crude product was purified by column chromatography (hexane/ethyl acetate 1:1). Purification afforded 200 mg (53 %) of the desired product 12a in yield along with 52 mg (15 %) of byproduct 13. The ratio 12a/13 was 80:20 (calculated from LC and¹⁹F NMR of the crude product).

¹H NMR (CDCl₃): 9.86 (1H, ddd, ³J_HF = 18.9, ⁴J_HF = 2.5, ⁶J_HH = 0.6, CHO), 7.64 (1H, dq, J = 1.4, 0.7, Imi), 7.52 (1H, td, J = 1.4, 0.6, Imi), 7.41-7.34 (9H, m, Tr), 7.18-7.11 (6H, m, Tr). ¹³C NMR (CDCl₃): 178.63 (dd, ²J_CF = 17.0, ³J_CF = 4.2, CHO), 157.62 (dd, ¹J_CF = 256.5, ²J_CF = 39.9, CF), 144.94 (dd, ¹J_CF = 244.7, ²J_CF = 35.6, CF), 141.31 (3C, Tr), 140.98 (bs, C2_Imi), 129.49 (6CH, Tr), 128.51 (3CH, Tr), 128.31 (6CH, Tr), 126.53 (dd, ³J_CF = 13.9, ⁴J_CF = 7.2, C5_Imi), 76.44 (s, 1C, Tr), C4_Imi is covered by signals of trityl group.¹⁹F NMR (CDCl₃): -152.4 (1F, dd, 120.8, 2.5), -168.4 (1F, dddt, 120.8, 18.9, 1.4, 0.7). HRMS (DCI): for C₂₅H₁₈F₂N₂O (M) calcd: 400.1387, found: 400.1389. Mp: 139-141 °C (ethylacetate). Elemental analysis for C₂₅H₁₈F₂N₂O calcd: 74.99 %C, 4.53 %H, 7.00 %N. Found: 74.91 %C, 4.48 %H, 7.02 %N.

4.8. (Z)-α,β-Difluoro-3-(1-trityl-1H-imidazol-4-yl)-propenal (12b)

To a solution of compound 11b (232 mg, 0.58 mmol) in anhydrous dichloromethane (10 mL) was added activated MnO₂ (501 mg, 5.77 mmol). The mixture was stirred at room temperature for 6 days until TLC indicated complete disappearance of starting material. The mixture was filtered, solvent was evaporated and crude product was purified by column chromatography. The crude product was purified by column chromatography (hexane/ethyl acetate 4/1) to give 76 mg (33 %) of the desired product 12b and 90 mg (42 %) of byproduct 13. The ratio 12b/13 was 40:60 (calculated from LC and ¹⁹F NMR of the crude product).

¹H NMR (CDCl₃): 10.58 (1H, dd, ³J_HF = 19.1, ⁴J_HF = 2.3, CHO), 7.53 (1H, dd, J = 2.9, 1.4, Imi), 7.42-7.34 (9H, m, Tr), 7.18-7.10 (7H, m, Tr, Imi). ¹³C NMR (CDCl₃): 184.71 (dd, ²J_CF = 15.4, ³J_CF = 7.7), 156.12 (dd, ¹J_CF = 262.8, ²J_CF = 16.2), 143.26 (dd, ¹J_CF = 254.6, ²J_CF = 12.7), 141.46 (3C, Tr), 141.08 (d, ⁴J_CF = 1.8, C2_Imi), 139.56 (d, ²J_CF = 23.9, C4_Imi), 129.56 (6CH, Tr), 128.55 (3CH, Tr), 128.37 (6CH, Tr), 124.37 (dd, ³J_CF = 8.9, ⁴J_CF = 3.3, C5_Imi), 76.32 (s, 1C, Tr).¹⁹F NMR (CDCl₃): -123.6 (1F, d, ³J_FF = 12.0), -159.4 (1F, dd, ³J_HF = 19.1, ³J_FF = 12.0). HRMS (DCI): for C₂₅H₁₈N₂F₂O (M) calcd: 400.1387, found: 400.1389. Mp: 136–137°C (ethyl acetate).

4.9. 2-Fluoro-1-(1-trityl-1H-imidazol-4-yl)-ethanone (13)

¹H NMR (CDCl₃): 7.70 (1H, d, J = 1.4, Imi), 7.44 (1H, d, J = 1.4, Imi), 7.39-7.33 (9H, m, Tr), 7.14-7.08 (6H, m, Tr), 5.29 (2H, d, ²J_HF = 47.4, CH₂F). ¹³C NMR (CDCl₃): 189.28 (d, ²J_CF = 15.8, CO), 141.45 (3C, Tr), 139.62 (s, C2_Imi), 137.46 (s, C4_Imi), 129.58 (6CH, Tr), 128.50 (3CH, Tr), 128.35 (6CH, Tr), 126.16 (d, ⁴J_CF = 4.2, C5_Imi), 83.55 (d, ²J_CF = 179.1, CH₂F), 76.31 (s, 1C, Tr). ¹⁹F NMR (CDCl₃): -235.7 (t, ²J_HF = 47.5). Mp: 193-194°C (ethyl acetate). HRMS (FAB⁺): for MH i.e. C₂₄H₂₀FN₂O calcd: 371.1560; found: 371.1553. Elemental analysis for C₂₄H₁₉FN₂O calcd: 77.82 % C, 5.17 % H, 7.56 % N. Found: 77.78 % C, 5.09 % H, 7.75 % N.

4.10 (E)-α,β-Difluorourocanic acid (6a)

Compound 12a (108 mg, 0.27 mmol) was dissolved in 8.0 mL of t-BuOH and 3.2 mL of 2-methyl-2-butene. Then the mixture was cooled to 0 °C and a solution of NaClO₂ (162 mg, 1.79 mmol) and NaH₂PO₄ (218 mg, 1.82 mmol) in 4.0 mL of H₂O was added. The resulting mixture was stirred at room temperature until TLC indicated complete disappearance of the aldehyde. Dichloromethane (80 mL) was added and the mixture was washed with water (2 × 10 mL), brine (2 × 10 mL) and dried over anhydrous Na₂SO₄. The mixture was filtered and evaporated. The residue was then washed with water (2 × 5 mL) and dissolved in 10 mL of CH₃COOH and 1.0 mL of concentrated HCl. The mixture was stirred at room temperature for 3 h. After this period the solvent was evaporated and the residue was washed subsequently with ethyl acetate (2 × 5ml) and 2 mL of H₂O. The residue was then dried on vacuum and 45 mg (96 %) of the final product 6a was obtained as a white solid.

¹H NMR (DMSO-d₆): 7.91 (bs, 1H), 7.69 (bs, 1H). ¹³C NMR (DMSO-d₆): 160.73 (dd, ²J_CF = 46.4, ³J_CF = 29.3, CO), 152.32 (dd, ¹J_CF = 254.2, ²J_CF = 46.4, CF), 137.63 (bs, C2_Imi), 137.47 (dd, ¹J_CF = 237.8, ²J_CF = 39.5, CF), 127.56 (d, ²J_CF = 31.9, C4_Imi), 121.89 (dd, ³J_CF = 13.7, ⁴J_CF = 6.0, C5_Imi). ¹⁹F NMR (DMSO-d₆): -138.90 (d, 1F, ³J_FF = 125.0), -160.88 (d, 1F, ³J_CF = 128.1). HRMS (ESI): for M-H i.e. C₆H₃F₂N₂O₂ calcd: 173.0163; found: 173.0164. Mp: decomposition between 250–270°C. Elemental analysis for C₆H₄F₂N₂O₂ calcd: 41.39 % C, 2.35 % H, 16.09 % N. Found 40.98 % C, 2.33 % H, 15.76 % N.

4.11 (Z)-α,β-Difluorourocanic acid (6b)

Compound 12b (70 mg, 0.18 mmol) was dissolved in 8 mL of t-BuOH and 3 mL of 2-methyl-2-butene. The solution was then cooled to 0 °C and a solution of NaClO₂ (160 mg, 1.78 mmol) and NaH₂PO₄ (214 mg, 1.78 mmol) in 4.0 mL of H₂O was added. The mixture was stirred at room temperature until TLC indicated the complete disappearance of aldehyde. Dichloromethane (80 mL) was added and the mixture was washed with water (2 × 10 mL), brine (2 × 10 mL) and dried over anhydrous Na₂SO₄. The mixture was filtered and evaporated. The residue was washed with water (2 × 5mL) and dissolved in 10 mL of CH₃COOH and 1.0 mL of concentrated HCl. After the mixture was stirred at room temperature for 3 h, the solvent was evaporated and the residue was washed with ethyl acetate (2 × 5 ml) and 2 mL of H₂O. Vacuum drying of the residue gave 11 mg (37 %) of the final product 6b as a white solid.

¹H NMR (DMSO-d₆): 8.42 (d, 1H, ⁴J_HF = 1.5), 7.99 (bs, 1H). ¹³C NMR (DMSO-d₆): 164.65 (dd, ²J_CF = 26.1, ³J_CF = 6.0, CO), 154.92 (dd, ¹J_CF = 250.8, ²J_CF = 21.8, CF), 141.81 (s, C2_Imi), 142.33 (dd,¹J_CF = 245.9, ²J_CF = 14.6, CF), 131.49 (d, ²J_CF = 35.3, C4_Imi), 125.10 (d, ³J_CF = 8.6, C5_Imi). ¹⁹F NMR (DMSO-d₆): -132.95 (bs, 1F), -141.99 (bs, 1F). HRMS (ESI): for M-H i.e. C₆H₃F₂N₂O₂ calcd: 173.0163; found: 173.0166. Mp: decomposition between 250–270°C. Elemental analysis for C₆H₄F₂N₂O₂ calcd: 41.39 % C, 2.35 % H, 16.09 % N. Found 41.16 % C, 2.35 % H, 15.81 % N.

Structures 1-6.

Stucture 13.

Scheme 1.